The gas giant Uranusseventh planet of the Solar System in order of distance from the Sun, has had a fascinating mystery in recent decades: a constant cooling of its thermosphere (the upper part of the atmosphere) compared to measurements made by the probe Voyager 2 in 1986: in just under 40 years the temperature went from 427 °C to 177 °C. A new study published in the scientific journal Geophysical Research Letters seems to finally provide one explanation to the phenomenon. Scientists have in fact discovered that temperature variation is closely related to pressure variation of the solar windwhich in turn acts on Uranus’ magnetosphere and therefore the amount of energy from the Sun that reaches the planet. This has important implications for the search for extra-solar planets since their search is based on mathematical models calibrated to what we can observe up close, namely the planets of the Solar System.
Because Uranus’ thermosphere is cooling
Until now, the rapid decline of around 250°C in Uranus’ upper atmosphere had remained unexplained. Thanks to a study published in the scientific journal Geophysical Research Letterswe now know that the one responsible for the cooling is the decrease in kinetic energy deposited every second by the solar wind on the magnetosphere of Uranus, which is the region of influence of the planet’s magnetic field, which is therefore correlated with the decrease in temperature.
The interaction of the weak solar wind with the magnetosphere has caused the latter to would expand over time. A larger magnetosphere acts as obstacle greater than the solar wind, which causes a decrease in energy flow that reaches the planet’s thermosphere, which is ultimately what heats it. Planets with sufficiently large magnetospheres, such as Uranus, might therefore also have thermospheres governed predominantly by solar wind (charged particles) rather than solar radiation (photons).
What Uranus’ atmosphere is like
Gas giant planets, such as Uranus, they do not have a solid surfacebut progressively denser gaseous layers as we approach the center of the planet. This means that the atmosphere of Uranus is not defined on the basis of the layer above the surface, but rather on the various layers that follow one another above, or just below, the threshold in which the atmospheric pressure is equal to that of the earth.
THE’atmosphere of Uranus is conventionally divided into three layers: the troposphere (up to 50 km above the Earth’s atmospheric pressure threshold), the stratosphere (between 50 and 4000 km) and the thermospherethe outermost layer that extends from 4000 km up to about twice the radius of Uranus. The thermosphere is made up of charged particlesions and electrons, including lo H ion3+(made up of three hydrogen atoms bonded together) which emits light radiation in the mid infrared detectable with ground instruments.
The first measurements of Uranus’ thermosphere were made by the Voyager 2 probe in 1986, which detected a temperature of approximately 427°C. Since then, Uranus’ thermosphere has been continuously monitored from Earth with infrared instruments which showed a rapid decline to the current values of around 127 °C.
What are the implications for extra-solar planets
In the study the authors highlight how this result has implications important implications for the search for extra-solar planets, in particular for those that have magnetospheres as large as those of Uranus. In these cases, stellar winds drive the temperature of the upper atmosphere, and existing mathematical models that neglect this energy input also tend to give a incorrect estimate of the quantity of hydrogen ions H3+ present. Therefore, if the search for an exoplanet is based on the search for H3+ and the expected amount is wrong, we may be missing potential exoplanets out there by underestimating the H emission3+ from exoplanets.
